Huanglongbing (HLB) is one of the most devastating diseases of citrus worldwide. It is associated with the fastidious bacteria ‘Candidatus Liberibacter asiaticus’ (CLas), ‘Ca. L. africanus’, and ‘Ca. L. americanus’. The bacteria are vectored by the Asian citrus psyllid, Diaphorina citri Kuwayama (ACP), and the African citrus psyllid, Trioza erytreae (Del Guercio). The disease causes yellow shoots, a blotchy mottle appearance of the leaves, dieback, and the eventual death of a tree (Bové, 2006). HLB associated with CLas vectored by ACP is jeopardizing the Florida citrus industry (Halbert et al., 2008) and threatens all citrus production in North America and the Caribbean basin.
Both CLas and ACP have host plants within the family Rutaceae outside of the Citrus genus (Deng et al., 2007b; Folimonova et al., 2009; Halbert and Manjunath, 2004; Peña et al., 2006). One plant species grown in Florida, Murraya paniculata (L.) Jack (a synonym for M. exotica L.; see USDA-ARS-NGRP, 2012), is commonly grown as a hedge and serves as a host of both ACP and CLas (Damsteegt et al., 2010; Deng et al., 2007a; Halbert and Manjunath, 2004; Hung et al., 2000). The production and trade of M. paniculata in Florida has been regulated since 2008 as a result of concerns of M. paniculata being a reservoir of CLas for commercial citrus (Clark, 2007). CLas transmission between Citrus sinensis (L.) Osbeck var. ‘Madam Vinous’ and M. paniculata by ACP has been demonstrated (Damsteegt et al., 2010). In the field, M. paniculata is infrequently infected with CLas, even in areas with high inoculum pressure (Walter et al., 2012). The importance of M. paniculata in citrus HLB epidemiology therefore deserves further exploration.
To help clarify the importance of M. paniculata relative to HLB epidemiology in commercial citrus in Florida, we investigated titers of CLas in M. paniculata and C. sinensis plants growing in the same field and in ACP from colonies maintained on infected M. paniculata or infected Citrus spp. Because CLas bacterial numbers in infected M. paniculata often are too low to be detected by 16S rDNA-based CLas-specific quantitative polymerase chain reaction (Li et al., 2006), we compared the detected copy numbers of an internal repeat region of two CLas prophage genes as a relative measure of CLas abundance.
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